1. Field of the Invention
The present invention relates to a rotation detecting device, a sheet feeding device, and an image forming apparatus that are used for copying machines, facsimiles, printers, and the like, and more particularly, to a rotation detecting device for detecting the remaining number of recording sheets in a tray of a sheet feeding device. Also, the present invention relates to a sheet feeding device and an image forming apparatus that have such a rotation detecting device.
2. Background Art
An image forming apparatus such as a copying machine, a facsimile, and a printer stores a large volume of recording sheets, and uses a sheet feeding device for transporting the recording sheets piece by piece (for example, refer to Japanese Patent Application Laid-Open Publication No. 3665201). The sheet feeding device shown in this patent document and the like includes: a tray that is removably attached to the main body of the image forming apparatus and stores a large volume of recording sheets; a push-up member that is rotatably attached at one end to the bottom surface of the tray; a sheet feeding roller that is provided in an upper portion of the tray, and transports recording sheets to a developing device piece by piece by being rotated by the rotation driving force of a motor as a driving source; a drive shaft that is rotatably provided in the tray with an interlocking push-up member attached thereto, the interlocking push-up member being disposed between the other end of the push-up member and the bottom surface of the tray; and a rotation detecting device that is provided in the main body, and has an output gear connected to the drive shaft when the tray is set in the main body.
The rotation detecting device includes: a case; the above-described output gear rotatably supported by the case; a motor as a driving source that is housed in the case and rotates the output gear in the forward rotation direction; multiple gears that transmit the rotation driving force of the motor to the output gear; and a detecting mechanism that detects the rotation of the output gear. The output gear and the multiple gears are in mesh with each other. The output gear and the multiple gears are formed of hard material such as synthetic resin containing glass fiber, various kinds of metal, sintering material (material formed and hardened by creating bonding between particles of non-metallic or metallic powder) in order to prevent wearing of the gears in mesh with each other.
The detecting mechanism includes: a rib part projecting from the surface of the output gear; a first fixed electrode mounted on the case; a second fixed electrode mounted on the case; and a detection circuit that detects rotation of the output gear by detecting contact state between the fixed electrodes. The rib part includes cam rib pairs provided at four locations at regular intervals in the circumferential direction of the output gear, i.e., provided at every 90 degrees on the output gear. That is to say, the rib part includes a total of four cam rib pairs. The cam rib pairs each includes an inner circumference cam rib that is a projection on the output gear and extends in the circumferential direction of the output gear, and an outer circumference rib that is provided on an outer side than the inner circumference rib, and is provided on the rear side in the forward rotation direction.
The first fixed electrode is formed with a thin metal sheet and is fixed to the case in a state where the first fixed electrode is opposed to and spaced away from the surface on which the above-described inner circumference rib and outer circumference rib of the output gear are provided. The second fixed electrode includes a pair of conductive spring pieces which are provided between the first fixed electrode and the above-described surface of the output gear and arranged spaced apart from each other along the radial direction of the output gear, and which are fixed to the case. One of the spring pieces is provided between the inner circumference rib and the first fixed electrode, and the other spring piece is provided between the outer circumference rib and the first fixed electrode. When coming into contact with the inner and outer circumference ribs, these spring pieces are pressed against and come into contact with the first fixed electrode by the inner and outer circumference ribs, respectively.
The detection circuit is electrically connected to the first fixed electrode, the pair of spring pieces of the second fixed electrodes, and the like in accordance with a pre-defined pattern. The detection circuit detects a rotation angle of the output gear, i.e., rotation of the output gear by detecting a state where each spring piece of the second fixed electrodes comes into contact with the first fixed electrode, or each spring piece of the second fixed electrodes is separated from the first fixed electrode.
In the above-described rotation detecting device, when the tray is inserted into the main body of the image forming apparatus, the output gear is coupled with the drive shaft. The rotation detecting device rotates the output gear and the drive shaft in the forward rotation direction by its motor, so that the interlocking push-up member attached to this drive shaft pushes the push-up member towards the sheet feeding roller. Subsequently, when a recording sheet on the push-up member comes into contact with the sheet feeding roller, the rotation detecting device stops the rotation of the output gear and the drive shaft. In this manner, the rotation detecting device rotates the output gear and the drive shaft until a recording sheet on the push-up member comes into contact with the sheet feeding roller, and then calculates the number of recording sheets in the tray by detecting the rotation angle of the output gear at this moment with the detection circuit of the detecting mechanism. The image forming apparatus displays the number of recording sheets detected by the rotation detecting device on, e.g., a displaying unit provided on the upper portion of the main body.
Because the output gear is formed of the above-described hard materials, in the rotation detecting device shown in the above-described patent document, a bearing which rotatably supports the output gear to the case, and the above-described inner and outer circumference ribs are easily worn out due to aged deterioration. Accordingly, in the above-described rotation detecting device, “misalignment” between the output gear and the case, and between the ribs and the spring pieces of the second fixed electrode is gradually increased due to aged deterioration, thus error in detecting the rotation angle of the output gear is gradually increased naturally due to aged deterioration. Thus a problem occurs in that the number of recording sheets in the tray cannot be accurately detected.